Zygarde-Complete's presence in the metagame is banworthy predominantly due to the following quality it possesses: Incredible bulk on both sides of the spectrum due to the drastic increase in base HP.

Given the fact that, once the complete form is activated, Zygarde-Complete has 216 base HP, it's fairly clear that Zygarde's bulk suddenly goes from great to absolutely amazing. Nothing ever to grace the OU tier before has had bulk even remotely comparable to Zygarde-Complete form. Now, standalone bulk is appreciated in general, but it's really what Zygarde-Complete can do in the context of the metagame as a byproduct of having this massive HP stat that makes it banworthy. It can run Dragon Dance and Coil sets with Thousand Arrows as the only necessary coverage move to become a sweeper with unmatched potential and longevity. It often utilizes Substitute or Rest in conjunction with Coil or Dragon Dance in order to combat opposing status and other support oriented means of counterplay, bar phazing (which is still usually only a temporary fix to a problem that lingers throughout the entirety of the game), and when Rest is used, it can comfortably be used alongside Sleep Talk and Thousand Arrows. Assuming that the complete form is activated, Zygarde-Complete is able to live many of the strongest attacks it will encounter in the metagame with ease, even Super Effective ones, and the calculations below will attest to this. While Zygarde-Complete certainly lacks the offensive presence that something like Mega Salamence might have had when it got quickbanned near the start of ORAS, it is very clear that Zygarde-Complete finds many opportunities to set-up without much stopping it and, therefore, it easily compensates for having only a fairly average base 100 base Attack. Given all of this, the only real forms of counterplay to Zygarde-Complete are Unaware Pokemon, such as Clefable and Quagsire, and Tapu Bulu, which is far too small of a list to be comfortable with. Overall, Zygarde-Complete's sweeping capabilities are absolutely absurd and it holds no place in the OU tier because of this.

As the above calculations show, trying to kill Zygarde-Complete is incredibly challenging, even for the strongest and most effective Pokemon in the current metagame.

Granted the aforementioned acknowledgement of Zygarde-Complete's unprecedented and practical level of bulk, a conclusion can be drawn that Zygarde-Complete is broken.

For the reason listed above, we have decided that Zygarde-Complete needs to be banned from the SM OU metagame. We have elected to ban the ability, Power Construct, and not the Pokemon because the ability allows a form change; this form change is different than past Pokemon banned due to a broken ability because of the fact the ability itself is what is broken, regardless of what posses it. Zygarde-Complete is undeniably a banworthy Pokemon and banning Power Construct allows us to eliminate that aspect from the game. Therefore, we are banning the ability Power Construct, effective immediately.